Summary The classical Bordetella species are genetically related obligate respiratory pathogens of mammals. Bordetella pertussis is an obligate human pathogen and the agent of pertussis, which remains a serious endemic disease. Bordetella parapertussis causes whooping cough in humans and some strains infect sheep. Bordetella bronchiseptica infects a wide range of nonhuman mammals, although strains have been isolated from humans. A significant hindrance to preventing pertussis is the lack of knowledge of B. pertussis basic biology and metabolism in its natural human host environment. Most bacterial pathogens require nutritional iron and must overcome host-imposed iron restriction for successful in vivo growth. Classical Bordetella species require TonB-dependent uptake systems to use their native alcaligin siderophore, xenosiderophores, and hemin as iron sources. Inflammation increases vascular permeability, promoting extravasation of plasma components onto the respiratory epithelial surface where Bordetella cells colonize. Therefore, plasma iron-containing proteins such as transferrin or hemopexin may provide iron to Bordetella. Human serum can serve as a relevant surrogate for the host environment in Bordetella studies. Our results revealed that B. pertussis exhibits robust growth in iron-depleted medium containing complement-inactivated human serum as the sole source of iron. Conversely, B. bronchiseptica showed a profound growth defect in the same human serum-containing medium, and this defect was relieved by addition of iron. Studies with B. pertussis mutants lacking their native siderophore or heme receptors or TonB still retained the ability to grow in human serum, revealing an apparently novel iron uptake mechanism. This project will use genetic and biochemical approaches to characterize this B. pertussis iron acquisition mechanism and identify the bacterial components as well as human serum iron sources involved. Surveys of the ability of different Bordetella species and strains to grow in human serum will determine the breadth of this iron uptake system and correlate its presence with the organism's host range. The research is important since it will characterize a new iron uptake system that may also be a human host-specific trait of B. pertussis.